Revisiting historical ocean surface temperatures

July 11th, 2011 by gavin

Readers may recall discussions of a paper by Thompson et al (2008) back in May 2008. This paper demonstrated that there was very likely an artifact in the sea surface temperature (SST) collation by the Hadley Centre (HadSST2) around the end of the second world war and for a few years subsequently, related to the different ways ocean temperatures were taken by different fleets. At the time, we reported that this would certainly be taken into account in revisions of data set as more data was processed and better classifications of the various observational biases occurred. Well, that process has finally resulted in the publication of a new compilation, HadSST3.

Figure: The new HadSST3 compilation of global sea surface temperature anomalies and the uncertainty.

HadSST3 not only greatly expands the amount of raw data processed, it makes some important improvements in how the uncertainties are dealt with and has a more Bayesian probabilistic treatment of the necessary bias corrections. That is to say that instead of picking the most likely factors and providing a single reconstruction, they perform a Monte Carlo experiment using a distribution of factors and provide a set of 100 reconstructions – the average and spread of which inform the uncertainties. This is a noteworthy approach and one which is likely to set a new standard for other reconstructions. (The details of the procedures are outlined in two new papers Kennedy et al, part I and part II).

One potential problem is going to be keeping the analysis up-to-date. Currently, HadSST2 (and HadCRUT3) use the real time updating related to NCEP-GTS. However, this service was scheduled to be phased out in March 2011 (was it?), in lieu of near-real time updating of the underlying ICOADS dataset. This is what HadSST3 uses, but unfortunately, the bias corrections in the modern period rely on being able to track individual fleets. For security reasons, data since 2007 has been anonymized (so you can’t tell what ship reported what data), and so the HadSST3 analysis currently stops at 2006. Apparently this is being worked on, so hopefully a solution can be found. Note that the ocean temperatures in the GISTEMP analysis use the Reynolds satellite SST data from 1979 and so are unaffected by the ICOADS security issue.

The ocean temperature history is obviously a big part of the global surface air temperature history and these new estimates will be used eventually in updates of the HadCRUT3 product. Currently HadCRUT uses HadSST2 and we can expect that to be updated soon.

Impacts

Obviously, when a new analysis is performed, it is interesting to see how it differs from previous ones. The differences between HadSST3 and HadSST2 are shown here:

and are important in a few key time-periods – the 1940s (because of issues highlighted previously), the 1860s to 1890s (more extensive data), and perhaps the last few years (related to more minor changes in technologies and corrections). The biggest difference around 1946-8 is just over 0.2ºC.